FIT3165 / FIT4165 COMPUTER NETWORKS
WEEK 10 – Physical Layer Transmission – Part 2
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10.1 ANALOG TRANSMISSION
10.2 BANDWIDTH UTILIZATION
10.3 TRANSMISSION MEDIA
ANALOG TRANSMISSION
ANALOG TRANSMISSION
• If digital transmission is desirable, it needs a low-pass channel;
• If analog transmission is the only choice if we have a bandpass channel.
• Converting digital data to a bandpass analog signal is traditionally called digital-to-analog conversion.
• Converting a low-pass analog signal to a bandpass analog signal is traditionally called analog-to-analog conversion.
Digital-to-Analog Conversion
• Digital-to-analog conversion is the process of changing one of the characteristics of an analog signal based on the information in digital data.
• Figure shows the relationship between the digital information, the digital-to-analog modulating process, and the resultant analog signal.
Amplitude Shift Keying
Binary ASK (BASK) Multilevel ASK Binary FSK (BFSK) Multilevel FSK
Phase Shift Keying
Binary PSK (BPSK) Quadrature PSK (QPSK) Constellation Diagram
Quadrature Amplitude Modulation
Bandwidth for QAM
Digital-to-analog conversion
Binary shift keying methods
s(t) = A * Sin (2π f t + θ) A = Amplitude
f = carrier frequency
θ = Phase Angle
s(t) = A * Sin (2π f t + θ)
A = Amplitude
f = carrier frequency θ = Phase Angle
“0” = 0 * Sin (2π f t + 0)
“1” = A * Sin (2π f t + 0)
Binary amplitude shift keying
“0” = A * Sin (2π f1 t + 0) “1” = A * Sin (2π f2 t + 0)
Binary frequency shift keying
Binary shift keying methods and constellation diagrams
“0” = A * Sin (2π f1 t + 0) “1” = A * Sin (2π f1 t + 180)
Binary phase shift keying Concept of a constellation diagram
s(t) = A * Sin (2π f t + θ) A = Amplitude
f = carrier frequency
θ = Phase Angle
Constellation diagrams for some QAMs
■ QAM used in asymmetric digital subscriber line (ADSL) and some wireless communication
■ combination of ASK and PSK
■ logical extension of QPSK
■ send two different signals simultaneously on same carrier frequency
■ uses two copies of carrier fc, one shifted by a phase angle of 90°
■ each carrier is ASK modulated
■ two independent signals over same medium
■ demodulate and combine for original binary output
Analog-to-Analog Conversion
• Analog-to-analog conversion, or analog modulation, is the representation of analog information by an analog signal.
• One may ask why we need to modulate an analog signal; it is already analog.
• Modulation is needed if the medium is bandpass in nature or if only a bandpass channel is available to us for communication wirelessly
❑ Amplitude Modulation
❑ Frequency Modulation
❑ Phase Modulation
Amplitude modulation
Analog-to-Analog Conversion
Frequency modulation
Phase modulation
BANDWIDTH UTILIZATION
Bandwidth utilization and Multiplexing
● In real life, we have links/communication channels with limited bandwidths.
● Hence, at times we need to combine several low-bandwidth channels to make use of one
channel with a larger bandwidth.
Multiplexing
● Multiplexing is the process where multiple channels are combined for transmission over a common transmission path.
● Full capacity of data transmission links are not always fully utilized
● To make efficient use of high-speed telecommunications lines, some form of multiplexing
● A common application of multiplexing is in long-haul communications.
Multiplexing – in long-haul communications.
Multiplexing
● Trunks on long-haul networks are high-capacity fiber, coaxial, or microwave links.
● These links can carry large numbers of voice and data transmissions simultaneously using multiplexing.
● Common forms of multiplexing are:
○ Frequency Division Multiplexing (FDM),
○ Wave Division Multiplexing (WDM)
○ Time Division Multiplexing (TDM)
■ Synchronous TDM
■ Statistical Time-Division Multiplexing
Multiplexing
Multiplexing
● Multiple n inputs / 1 outputs on 1 physical line
● Multiplexing allows several transmission sources to share a larger
transmission capacity
● The link can carry multiple channels of data
● Common on long-haul, high capacity, links
Frequency Division Multiplexing
● Division of a transmission link into multiple channels by splitting the frequency band into multiple slots.
● Used when useful bandwidth of the link is greater than required bandwidth of individual signals to be transmitted.
● Each signal is modulated on a different carrier frequency.
● Carrier frequencies are centered at each signal BW and typically separated by guard bands so that signals do not overlap.
● An FDM receiver uses filters, one per slot, to separate the individual channels, each of which is separately demodulated to extract the signal.
Multiplexing – Example
Assume that a voice channel occupies a bandwidth of 4 kHz. We need to combine three voice channels into a link with a bandwidth of 12 kHz, from 20 to 32 kHz. Show the configuration, using the frequency domain. Assume there are no guard bands.
Synchronous Time Division Multiplexing
Time slots on a shared medium are assigned to devices on a fixed, predetermined basis
TDM – Example
Telephone companies implement TDM through a hierarchy of digital signals, called digital signal (DS) service or digital hierarchy. Figure 7.51 shows the data rates supported by each level. The commercial implementations of these services are referred to as T lines.
❑ DS-0 service is a single digital channel of 64 kbps. ❑ DS-1 is a 1.544-Mbps service.
❑ DS-2 is a 6.312-Mbps service.
❑ DS-3 is a 44.376-Mbps service.
❑ DS-4 is a 274.176-Mbps service.
TDM Types – Slot Comparison
Wavelength- Division Multiplexing
• FDM with multiple beams of light at different frequencies
• WDM carried over optical fiber links uses λ wavelength
– commercial systems with 160 channels of 10 Gbps
– lab demo of 256 channels at 39.8 Gbps
• Architecture similar to other FDM systems
– multiplexer consolidates laser sources (1550nm) of different λ ‘s for transmission over single fiber
– Optical amplifiers amplify all wavelengths
– Demux separates channels at the destination
• There are two types of WDM, namely:
– Coarse WDM (CWDM) – here the wavelengths are spaced well apart
– Dense WDM (DWDM) – here larger number of wavelengths are more closely spaced
Spread Spectrum
• In spread spectrum, we also combine signals from different sources to fit into a larger bandwidth, but our goals are somewhat different.
• Spread spectrum is designed to be used in wireless applications (LANs and WANs). In these types of applications, we have some concerns that outweigh bandwidth efficiency. In wireless applications, all stations use air (or a vacuum) as the medium for communication.
• Stations must be able to share this medium without interception by an eavesdropper and without being subject to jamming from a malicious intruder.
Frequency Hopping Spread Spectrum (FHSS)
Bandwidth Sharing
Direct Sequence Spread Spectrum (DSSS)
Spread Spectrum
• Spread-spectrum techniques are methods by which a communicating signal generated with a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth.
• The practice of spreading the transmitted signal is done to occupy the frequency spectrum available for transmission.
• The spread spectrum transmission uses spreading codes to spread the signal out over a wider bandwidth than what would normally be required.
Frequency hopping spread spectrum (FHSS)
• Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver.
• FHSS is a wireless technology that spreads its signal over rapidly changing frequencies.
FHSS cycles
Direct Sequence Spread Spectrum (DSSS)
• Direct Sequence Spread Spectrum (DSSS) is a spread spectrum technique whereby the original data signal is multiplied with a pseudo random noise spreading code. This spreading code has a higher chip rate (this is the bitrate of the code), which results in a wideband time continuous scrambled signal.
• DSSS significantly improves protection against interfering (or jamming) signals, especially narrowband and makes the signal less noticeable.
• It also provides security of transmission if the code is not known to the public.
TRANSMISSION MEDIA
TRANSMISSION MEDIA
• In this section, we discuss transmission media. Transmission media are actually located below the physical layer and are directly controlled by the physical layer.
Figure: Transmission media and physical layer
Guided Media
• Guided media, which are those that provide a conduit from one device to another, include twisted-pair cable, coaxial cable, and fiber-optic cable.
• A signal traveling along any of these media is directed and contained by the physical limits of the medium. Twisted-pair and coaxial cable use metallic (copper) conductors that accept and transport signals in the form of electric current.
• Fiber-optic cable is a cable that accepts and transports signals in the form of light.
Twisted-Pair Cable
❖ Performance
❖ Applications
❑ Coaxial Cable
❖ Performance
❖ Applications
❑ Fiber-Optic Cable
❖ Propagation Modes
❖ Performance
❖ Applications
Guided Media
Twisted-pair cable
Coaxial cable
Guided Media
Optical fiber
Bending of light ray
Optical Fiber – Modes
Unguided Media
• Unguided media transport electromagnetic waves without using a physical conductor.
• This type of communication is often referred to as wireless communication.
• Signals are normally broadcast through free space and thus are available to anyone who has a device capable of receiving them.
❑ Radio Waves
❑ Microwaves
❑ Infrared
Electromagnetic spectrum for wireless communication
Frequency Bands
● So far we have discussed
○ Analog transmission
○ Digital Transmission
○ Guided Media
○ Unguided Media
● Next week
○ LAN overview
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